Make the Crumb Topping: In a food processor, combine the butter with the sugar, flour, baking soda, and salt, and process until the mixture forms small crumbs. Transfer to a plate and press into 1/2-inch clumps. Refrigerate for 15 minutes, until firm.

Make the Cake: In a small saucepan, cook the butter over moderate heat until the solids turn brown, 5 minutes. Immediately scrape the butter and solids into a shallow bowl and freeze until firm but not hard, 15 minutes.

Preheat the oven to 350 degrees F. Spray a 9-inch springform pan with baking spray and line the bottom with parchment paper. Spray the paper. In a bowl, whisk the flour with the baking powder, baking soda, salt, and nutmeg. Scrape the firmed brown butter into the bowl of a standing electric mixer fitted with the paddle. Add the sugar and beat at moderate speed until light and fluffy, 5 minutes. Add the eggs and vanilla and beat until smooth. Working in 3 alternating additions, beat in the dry ingredients and the sour cream, scraping down the bowl occasionally.

Scrape the batter into the prepared pan and scatter the crumbs on top. Bake in the center of the oven for 45 minutes, until the cake is golden and a skewer inserted in the center comes out clean. Let cool.

Remove the ring and transfer the cake to a plate. Peel the paper off the bottom of the cake before serving.